Autism spectrum disorders (ASD) are pervasive developmental disorders characterized by abnormalities in a variety of social, communicative, and emotional behaviors, which occur in 0.67 percent of the population (Centers for Disease Control and Prevention, February 2007 report). Because ASD is not diagnosed reliably before the second or third year of life, the best way to investigate development of the disorder involves studying infant siblings of children with ASD (High-Risk infants), who have a significantly increased risk (10- to 20-fold) of developing ASD. Although only a minority (~9 percent) of High-Risk infants is predicted to ultimately develop ASD, the remaining ~91 percent can nonetheless be expected to exhibit symptoms of the broader autism phenotype (i.e., sub-clinical atypicalities that are seen in the relatives of individuals with ASD). The current proposal studies High-Risk infants from ages 6 to 36 months, tracking their development in comparison to control infants to look for abnormalities associated with ASD. Taking a neuro-developmental approach and employing a variety of techniques, the proposal tests hypotheses regarding specific brain areas/systems we believe may be abnormal early in the development of ASD, as follows: 1) At 6 months, we test the possibility of abnormal processing within the Magnocellular subcortical visual pathway, by using perceptual techniques to assess M pathway function (vs. Parvocellular pathway function as a control). 2) At 7 months, we test the possibility of abnormal motion processing within the dorsal cortical pathway, by measuring perceptual responses to biological motion stimuli (vs. form stimuli as a control). 3) At 7 and 10 months, we test the possibility of abnormal face processing within the ventral cortical pathway, by measuring perceptual responses and event related potentials (ERPs) elicited by face stimuli (vs. object stimuli as a control), as well as positive vs. negative facial expressions of emotion. 4) At 18 months, we use behavioral and ERP methods to test for abnormalities in social/emotional processing, which is likely to involve the amygdala and prefrontal cortex. 5) Over the course of the study, several standardized tests are administered to assess social, communicative, and cognitive skills, including those that are used to diagnose ASD. We have great hope that these studies will shed light on the neuro-developmental origins of ASD (and the broader autism phenotype), and aid in early diagnosis of the disorder. Autism spectrum disorders (ASD) are developmental disorders characterized by differences in a variety of social, communicative, and emotional behaviors, which occur in 0.67 percent of the population. Because ASD is not diagnosed reliably before the second or third year of life, the only current way to study development of the disorder involves working with infant siblings of children with ASD, who have a significantly increased risk (i.e., 10- to 20-fold) for developing ASD. Our laboratory group works with these infants from ages 6 to 36 months, tracking their development to look for atypicalities that may be early signs of ASD, which we believe will shed light on how ASD develops, as well as aid in early diagnosis of the disorder.